Chapter 12 Lactobacilli, corynebacteria and propionibacteria
Lactobacilli are saprophytes in vegetable and animal material (e.g. milk). Some species are common animal and human commensals inhabiting the oral cavity and other parts of the body. They have the ability to tolerate acidic environments and hence are believed to be associated with the carious process.
The taxonomy of lactobacilli is complex. They are characterized into two main groups: homofermenters, which produce mainly lactic acid (65%) from glucose fermentation (e.g. Lactobacillus casei), and heterofermenters, which produce lactic acid as well as acetate, ethanol and carbon dioxide (e.g. Lactobacillus fermentum). L. casei and Lactobacillus rhamnosus, Lactobacillus acidophilus and the newly described species, Lactobacillus oris, are common in the oral cavity. It should be noted that the taxonomy of lactobacilli is under constant revision.
Gram-positive coccobacillary forms (mostly bacillary), α- or non-haemolytic, facultative anaerobes. These organisms ferment carbohydrates to form acids (i.e. they are acidogenic) and can survive well in acidic milieu (they are aciduric); they may be homofermentative or heterofermentative. The question as to whether they are present in carious lesions because they prefer the acidic environment, or whether they generate an acidic milieu and destroy the tooth enamel, has been debated for years (the classic ‘chicken and egg’ argument).
Lactobacilli are also major constituents of the vaginal flora and help maintain its low pH equilibrium. Recently, the beneficial role of lactobacilli in maintaining the homoeostasis of the intestinal flora has been recognized, and ‘lactobacillus-laced’ food items have gained popularity among the health-conscious public.
Lactobacilli grow under microaerophilic conditions in the presence of carbon dioxide and at acidic pH (6.0). Media enriched with glucose or blood promote growth. A special selective medium, tomato juice agar (pH 5.0), promotes the growth of lactobacilli while suppressing other bacteria. Identification is by biochemical reactions.
Lactobacilli are frequently isolated from deep carious lesions where the pH tends to be acidic. Indeed, early workers believed that lactobacilli were the main cariogenic agent (a theory that has been disproved), so much so that the number of lactobacilli in saliva (the lactobacillus count) was taken as an indication of an individual’s caries activity. Although this test is not very reliable, it is useful for monitoring the dietary profile of a patient because the level of lactobacilli correlates well with the intake of dietary carbohydrate.
The genus Corynebacterium contains many species that are widely distributed in nature. These Gram-positive bacilli demonstrate pleomorphism (i.e. coccobacillary appearance) and are non-sporing, non-capsulate and non-motile. In common with Mycobacterium and Nocardia spp., they have a cell wall structure containing mycolic acid. A number of species are important human pathogens and commensals. The sometimes fatal upper respiratory tract infection of childhood diphtheria is caused by Corynebacterium diphtheriae. It is important to distinguish this, and other pathogens within the genus, from commensal corynebacteria.
Pleomorphic, Gram-positive, club-shaped (tapered at one end) bacilli, 2–5 µm in length, arranged in palisades. They divide by ‘snapping fission’ and hence are arranged at angles to each other, resembling Chinese characters. The rods have a beaded appearance, with the beads comprising an intracellular store of polymerized phosphate. The granules stain metachromatically with special stains such as Neisser methylene blue stain (i.e. the cells are stained with blue and the granules in red).